An offset amount between heads is calculated by positioning a head in a designated track under an ON track servo control, obliquely writing test data by moving the head in a radial direction of the magnetic recording medium at a designated moving speed K to cross the designated track and at least one track adjacent to the designated track according to a write start signal, reading the oblique test data by the read head from the adjacent track under the ON track servo control, detecting a peak position of a read voltage by using a read voltage characteristics of the read signal in the adjacent track and calculating the offset amount between the heads by detecting a scan time up to the peak position of the read voltage.